Re: Asynchronous Append on postgres_fdw nodes.

Etsuro Fujita <etsuro.fujita@gmail.com>

From: Etsuro Fujita <etsuro.fujita@gmail.com>

To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>

Cc: Justin Pryzby <pryzby@telsasoft.com>, Andrey Lepikhov <a.lepikhov@postgrespro.ru>, "movead.li" <movead.li@highgo.ca>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2021-03-08T05:30:40Z

Lists: pgsql-hackers

On Tue, Nov 17, 2020 at 6:56 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> * I haven't yet added some planner/resowner changes from Horiguchi-san's patch.

The patch in [1] allocates, populates, frees a wait event set every
time when doing ExecAppendAsyncEventWait(), so it wouldn’t leak wait
event sets.  Actually, we don’t need the ResourceOwner change?

I thought the change to cost_append() proposed in his patch would be a
good idea, but I noticed this:

+                   /*
+                    * It's not obvious how to determine the total cost of
+                    * async subnodes. Although it is not always true, we
+                    * assume it is the maximum cost among all async subnodes.
+                    */
+                   if (async_max_cost < subpath->total_cost)
+                       async_max_cost = subpath->total_cost;

As commented, the assumption isn’t always correct (a counter-example
would be the case where all async subnodes use the same connection as
shown in [2]).  Rather than modifying that function as proposed, I
feel inclined to leave that function as-is.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAPmGK14wcXKqGDpYRieA1ETgyj%2BEp5ntrGVD%3D29iESoQYUx9YQ%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAPmGK17Ap6AGTFrtn3%3D%3DPsVfHUkuiRPFXZqXSQ%3DXWQDtDbNNBQ%40mail.gmail.com

Commits

Improve comments for trivial_subqueryscan().
- 4a8a5dd7f59c 15.0 landed
Disable asynchronous execution if using gating Result nodes.
- ebb790241523 14.3 landed
- 5c854e7a2c8a 15.0 landed
Allow asynchronous execution in more cases.
- c2bb02bc2e85 15.0 landed
Doc: Further update documentation for asynchronous execution.
- eab81953682d 14.0 landed
Fix rescanning of async-aware Append nodes.
- f3baaf28a6da 14.0 landed
Doc: Update documentation for asynchronous execution.
- 15fcd33e0694 14.0 landed
Fix EXPLAIN ANALYZE for async-capable nodes.
- a363bc6da96b 14.0 landed
Minor code cleanup in asynchronous execution support.
- bb684c82f733 14.0 landed
Adjust input value to WaitEventSetWait() in ExecAppendAsyncEventWait().
- a8af856d3257 14.0 landed
Add support for asynchronous execution.
- 27e1f14563cf 14.0 landed